Climatic variations. Their extent and causes

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THEIR EXTENT .ANIL CAUSES

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UNIVERSITY, GLASGOW.

MEXICO.

IMl'KENTA V FOTO'I'II'IA 1>K LA SKCIIKTARf A IJE FOMENTO
Callej6n de Betlemitas niimero 8.

1906

THEIR EXTENT

CAUSES

CT. W_

IR,. S.

UNIVERSITY, GLASGOW.

MEXICO.

IMPUKNTA Y FOTOTIPIA DE LA SECRETAltfA DE FOMENTO

Callejdn de Betlemitas niimero 8.

1906

CONTENTS.

I. — Introduction.

H-— The General Uniformity of Climate in the Past.
III. — Exaggerated Estimates of Climatic Changes.

(a) The evidence of Fossil Corals.

(b) The evidence of Fossil Floras.

IV. — Glaciations due to Local Climatic Variations.
V. —Causes of Climatic Variations.

1 The Elevation Theory.

2 The Ohliquity of the Ecliptic.

3 Variations in the Carbonic Acid content of the Atmosphere.

(a) The Oceanic Control.

(b) The Evidence of Palaeontology.

(c) Objections to the Theory; (1) Refrigeration of climate not univer-
sal; (2) Non coincidence of Dates.

4 Change in temperature gradient of the Atmosphere,
o Changes in Atmosphere Circulation.

283907

CLIMATIC VARIATIONS, THEIR EXTENT AND CAUSES.

BY PROFESSOR

J. W. GREGORY F. R. S. UNIVERSITY, GLASGOW.

I. — INTRODUCTION.

The past variation of climate is an attractive study, ns it
controls so many questions in geology, geography and meteoro-
logy. I Jut subject is of especial difficulty, as it deals with the
action of complex chemical and physical processes working
under conditions and on material®, which can be •estimated only
by the freest speculation. The question may be approached a
priori by consideration of the evolution of the atmosphere, as
suggested by general chemical probabilities; or we may deter-
mine from the sedimentary rocks the strength and nature of the
geographical agencieis -that formed them; or we may examine
the indirect evidence given by fossils as to the climates under
which they lived. The fact of marked local variations in climate
is abundantly proved; and it will probably be equally agreed,
that there is no evidence known to the geologist of any pro-
gressive refrigeration of the earth. The idea of the secular co-
oling of the earth is deeply impressed on our terminology ; but
geological principles are independent of the theory. The terms
suggested by it may always be retained from their historic in-
terest and convenience, as we still speak of the rising of the sun.
Responsibility for the belief in the secular cooling of the earth
rests Avith the astronomers and physicists, from whom geolo-
gists have accepted it.

Local variations in climates are abundantly established by
the former glaciation of temperate regions, the once greater ex-
tension of glaciers in tropical regions, and the frequent growth

6 J. W. GREGORY.

of reef-building corals outside their present geographical limits.
But we need not unnecessarily increase the difficulties of the
problem by accepting the world-wide range of great climatic
changes without convincing evidence. D'r. Ekholm takes as the
starting point of his valuable paper, the ground that "the inqui-
" ries of modern geology unanimously indicate that all great
" climatic changes have occurred simultaneously on the whole
" earth."1

But geological opinion isi by no means unanimous on this
question; and the major climatic variations were world-wide
in their influence. The amplest evidence in support of the view
that a colder climate was once universal is supplied by the
Pleistocene glaciations; and it is certain that at one part or
another of the Pleistoncene period, the glaciers of many distant
parts of the world were much larger, and that wide areas in
the north temperate zones were overwhelmed by glacial condi-
tions. But there appears to be a steadily growing opinion that
the glaciers of the different glacial centres did not attain their
greatest development at the same time. Thus the glaciation of
Greenland1 is now at its maximum; at an earlier period of the
Pleistocene, Labrador was covered by an ice-sheet, which dwin-
dled as that of Greenland developed; and the glaciation of the
Canadian Rocky Mountains was probably still earlier than that
of Labrador. Similarly in Europe, the conditions of preserva-
tion and general aspect of the glacial deposits suggest that the
culmination of the Norwegian glaciation was somewhat later
than that of the British Isles.

II. — THE GENERAL UNIFORMITY OF CLIMATES IN THE PAST.

The first striking fact in the geological history of climate is
that the present climate of the world has been maintained since
the date of the earliest, unaltered, sedimentary deposits. The
oldest sandstone® of the Scotch Hihglauds and the English Long-
mynds, show that in pre-Cambrian times the winds had the
same strength, the rain drops were of the same size, and they

1 Dr. Nils Ekholm. "On the Variations of the Climate of the Geological and His-
torical Past and their Causes." Q. J. It. M<-t. Soc. XXVII, 1001, p. 3.

CLIMATIC VARIATION'S. 7

fell with the same force as at the present day. The evidence
of palaeontology proves that the climatic zones of the earth
have been concentric with the poles as far back as its records
go; the salts deposited by the evaporation of early Palaeozir
lagoons show that the oldest seas contained1 the-sanie materials
in solution as the modern oceans; and glaciatious have recurred
in Arctic, and, under special geographical conditions, also in
temperate regions, at various periods throughout geological
time.

The mean climate of the world has been fairly constant;
though there have been local variations, which have led to the
development of glaciers in regions now ice-free, at various parts
of the geological scale. That there has been no progressive
chilling of the earth since the date of the oldest known sedi-
mentary rocks, is shown by their lithological characters, and by
the recurrence of glacial deposits, some of which were laid
down at low levels, at intervals throughout geological time.
Thus remnants of a series of glacial deposits, which are pro-
bably pre-Cambrian, occur in a series of localities around the
Arctic Zone;1 fragments of this early, circum-Arctic glacial
chain occur in the North of Norway, as described by Reusch and
Strahan;2 in Spitsbergen;3 as some boulder beds, the descrip-
tions of which are -suggestive of glacial formation, on the
Coppermine River, and in Labrador, where however, according
to A. P. Low, they may be Cambrian; and finally on the north-
ern coasts of Siberia, near the estuary of the Lena. The Cam-
brian System contains an extensive series of glacial deposits,
discovered by Mr. Howchin4 running north and south through
South Australia, between the latitudes of 32° and 35° S. ; and
as these Cambrian tilk are inter-stratified with marine rocks,
they were probably formed about sea level.

The next proved glacial period is the Upper Carboniferous

1 \V. Gregory. Quart. Journ. Geol. Soc. Vol. LIII, 1897, p. 155.

•2 A. Strahan. "The Raised Beaches and Glacial Deposits of Varanger Fiord." Ibid.
Vol. LIII, 1897, pp. 147-153.

3 The Pro-0'arnbrian Glacial bed in Spitsbergen was referred to by Xordenskjold.
I accidcntly rediscovered it at Fox Point on Bell Sound in 1896, and sketched the best
exposed section: Quart. Journ. (ieol. Soc. Vol. LIV, 1898, p. 216.

4 Brief Reference to these Cambrian Glacial Deposits is given in Mr. W. How-
chin's paper "The Geolcgy of the Mount Lofty Ranges." Pt. I., Trans. R. Soc. S.
Austral., Vol. XXVIII, 1904, pp. 259, 278 and pi. XLIII.

8 J. W. GREGORY.

and perhaps Permian, as proved by the glacial deposits of In-
dia, South Africa, Australia, and South America. They vere
originally assigned, in Africa and Australia, to the Trias, and
subsequently to the Permian ; and the Permian age of the South
Africa glacial deposits still asserted by some geologists. But
according to Mr. Seward1 the glacial deposits at Vereniging —
which according to one theory are redeposited glacial material
and would therefore be the latest of the South African glacial
beds, — are Upper Carboniferous, and that is the age of the best
known and most extensive of the glacial deposits of south-eas-
tern Australia. The Upper Cretaceous has some evidence of gla-
ciation in the Northern Hemisphere; for1 the ocurrence of drift
ice is the most probable explanation of the boulders found in
the British chalk; and Professor Gar-wood found a glaciated
pebble on Bunting Bluff in Spitsbergen, in some conglomerates
which are upper Cretaceous, or Lower Cainozoic.2 With the ex-
ception of such scraps of evidence there is no convincing proof
of low level glacial action until we reach the Pleistocene.

III. — EXAGGERATED ESTIMATES OF CLIMATIC CHANGES.

The range of climatic variations in the past has been often
greatly exaggerated, thereby leading to the apparent necessity
for -revolutionary changes in former meteorological conditions.
But the climatic changes we have to explain, appear to have
been either local in area or moderate in degree.

The opinion, that there have been fundamental changes in
climate, is based mainly upon the evidence of former glacia-
tions, and on the 'supposed existence of tropical climates in the
Arctic regions. That tropical or sub-tropical conditions once
prevailed within the Arctic circle is affirmed on the reported
occurrence there of fossil coral reefs and tropical vegetation. I
have previously quoted evidence to show that this view is
greatly exaggerated/"' One notice of that paper dix-cribed its
views as "tres bardie,'' but C am not award of any refutation

1 A. ('. Seward. "Fossil Floras of Cape Colony." Annals S. Africa Museum, Vol. IV,
1't. I, 1903, p. 101.

2. Quart. Journ. Gecl. Soc. Vol. L1V, 18'J8, p. 217.

3 Some Problems of Artie Geology. II. Former Arctic Climates.

CLIMATIC VARIATIONS. 9

of its conclusions. The idea of the former tropical condition of
Greenland is still confidently asserted. Thus Dr. Ekholm1 refers
to the nearly tropical climate that prevailed in the Arctic re-
gions during the Cretaceous age, when lie estimates that the
mean temperature was 36° F. higher than during the Pleistoce-
ne. But so far as I know the evidence, there is mTproof that the
Arctic regions ever had a subtropical or even a warm temperate
climate.

(a) The Evidence of Foxxil ('orals.

The Arctic Ocean has been described as having been a coral
sea in Silurian and Carboniferous times. This view led to Blan-
det's suggestion — well known by its advocacy by Sir John Mur-
ray— that in Palaeozoic times light and heat were equally dis-
tributed throughout the world; and also to the theories that the
heat from the sun is diminishing owing to the smaller size of
the sun, as suggested by Helmholtz, or to its lower intensity, as
advocated by Dubois. But the fossil faunas of the Arctic seas
all show the dwarfing effect of unfavourable conditions, Avhen
compared to the contemporary faunas in the seas to the south.

("orals of reef building genera have lived in the Arctic regions ;
but I have seen no Arctic specimens larger than nodules, which
could have grown ir a cool sea. The asserted existence of Arctic
coral reefs in Silurian times was based on a collection made in
(Irinnell Land, which is now in the British Museum. But the
specimens show nothing more than the growth of small nodular
corals, such as may have grown in a temperate sea; Palaeozoic
corals ha.ve also been found in the Timan-Urals and in the Silu-
rian rocks of theNew Siberian Inlands; but in both cases the
evidence shows that the coral faunas were stunted in compari-
son with those of the contemporary seas to the south. Nu-
merous simple and simply branched corals, associated with
thick growths of calcareous algae, grow to-day in the northern
seas. Dead branches of Lophohelia are so common on one bank
in the Christiania Sound (hit. 58° N. ) that it has been descri-
bed ais a Pleistocene coral reef. Small nodules of corals, of reef

1 Ekholm. Op. fit. pp. 25, 2C.

Climatic Variations. — 2

10 -T. W. GREGORY.

building genera, such as Plesiastraea, live at present in the cold
seas of Southern Australia, far to the south of the region of
coral reefs.

Hence I feel justified in repeating the view expresed in 1S!)T,
that the evidence of the fossil corals from the Silurian rocks
of Greenland Britain shows "that there was almost as great
" a difference between the temperature of the sea in the areas
" as there is to-day."1

The evidence of the fossil corals is supported by that of the
Arctic marine faunas of all geological periods. Their most strik-
ing characteristics in the past are their characteristics of to-
day, and show "that all through geological time the northern
" faunas have lived under the blight of Arctic barrenness."2

(b) The Evidence of the A'o.s-.s-// I-'lonis.

The fossil floras of the Arctic as identified by Herr, have been
used as the basis of the attractively sensational theory that
Greenland enjoyed a tropical climate in Miocene times and a
tropical or sub-tropical climate in Cretaceous times. But the
evidence so far adduced appears to be quite imsuflicient to jus-
tify this view. The most characteristically tropical of the plants
. claimed to occur in Greenland are the palms; but the fossil Arc-
tic palms have now been dismissed as based on erroneous identifi-
cations. Much weight has also been attached to -some fossil tree-
ferns cf the genus Dicksonia, from the Cretaceous of Greenland.
But the best known living species of that genus is Dicksonia
Antarctica, which occurs in southern Xew /calami ; and Dickso-
nia also lives on the high "Snowy Plains" of the Victorian High-
lands, where it is sometimes buried under snow for four or five
months in the year. Hence the existence of fossil treeferns.
especially of the genus Dicksonia, would certainly not imply
tropical conditions. Heer's identifications have been contemp-
tuously rejected by many later botanists, including D'r. Robert
Brown, Dr. Starkie Gardner and Professor Xathorst. Most of
Heer's determinations were based upon leaves, which give no

1 Op. Cit, Nature, Vol. LVI, 1S!>7, i>. 862.

2 Ibid. p. 352.

CLIMATIC VARIATIONS. H

data for generic identification. Nor does tire existence of leaf
beds in the Arctic prove anything more than local geographical
changes; for leaves grow with remarkable rapidity and luxu-
riance within the Arctic circle, under the influence of the con-
tinuous day light of glimmer. That dense foliage grows upon
the moraines of Alaska is well known from the photographs,
taken upon the Malaspina, Glacier, published by I. P. Russell ;*
and in the same district, forests of fir trees growing on mo-
raines, are being now transported by the Alaskan glaciers.

The fossil tree trunks in Arctic coal seams would supply better
evidence of a change of climate than the fossil leave®, if there
were evidence to prove that the trees had grown in situ. The
view that the three months darkness: of winter would be fatal
to tree growth is now recognised as untenable; but its a fact
that foirstx do not occur north of 70°, although fossil tree trunks
have been found beyond that latitude. Rut these tree trunks were
probably carried north as drift wood.

Robert Brown has described the Disco plant beds and come
to the definite conclusion that the plants had not grown in situ.
Baron von Tol has published photographs of plant beds associa-
ted with ancient ice in northern Siberia ; but his photographs
show the roots of nothing larger than shrubs. In 1896 I had
occasion to mine some hundredweights of coal from the seam
at Advent Bay, Spitsbergen (hit. 78° 15' N. ), and the section
exposed gave no evidence that the coal had been formed from
vegetation that had grown in situ.

In many places the Arctic shores are white with a litter of
pine, fir, and larch logs, which have been floated down the Sibe-
rian rivers, drifted across the Arctic Ocean and been thrown
upon the shores.2 These accumulations of drift wood become
covered by the growth of moss, saxifrages and Arctic wrillows;
and if then buried beneath sheets of sediment, would form Arc-
tic coal seams, made from timber that had grown in Central
Asia.

The palaeobotanical evidence that the Arctic regions had a

1 C. Russell. "Second Expedition to Mount St. Elias." 13th. Ann. Rep. U. S. Geol.
Surv., 1893, pi. XIV.

'2 A photograph shewing one of these timber strewn beaches has been published
>'n the "Voyage de la Manche," plate V, 1894.

12 -T. W. GREGORY.

tropical or subtropical climate in the Cretaceous is inadequate;
and it is contradicted by the Palaeozoological evidence of the
contemporary marine deposits. The Cretaceous marine beds in
Greenland have a stunted fauna, which has no tropical or sub-
tropical characters. The British chalk sea was sufficiently cold
for drift ice to carry boulders as far south as London and its
fauna is decidedly non tropical. The chalk sea was of moderate
depth; but its Crinoids were small and scarce; its corals were
small and simple; and its mollusca indicate a cooler sea than do
the Hippiirites, etc., of the Mediterranean beds. In the Lower
Cretaceous beds of British Isles, there are abundant shallow sea
and shore deposits; but there are no coral reefs, and the general
aspect of the fauna indicates a sea decidedly colder than that
of the Jurassic. The British Cretaceous marine deposits indi-
cate the prevalence of a cool temperate, and those of Greenland
an Arctic climate, in the period when, on the unreliable eviden-
ce of fossil leaves, we are asked to believe the conditions in
Greenland were tropical or subtropical.

The palaeontological evidence at present available does not
throw on us the burden of explaining why the Arctic had a tro-
pical climate, for it simply contradicts assertion as a matter
of fact.

IV. — GLACIATION DUE TO LOCAL CLIMATIC VARIATIONS.

The second line of evidence used to prove intense, widespread
climatic changes is the oeurrence of glacial deposits in the tem-
perate zones, and the greater extensions of tropical glaciers.
But this evidence has also been used to indicate more extreme
changes than are necessary to explain the facts. Thus it appears
to be sometimes considered that the glacial beds of South Afri-
ca, India and Australia, prove that in one epoch of the Upper
Paleozoic the whole area of the Indian Ocean, from 30 N. lat.
in India to more than 40° S. in Tasmania wa;S undergoing gla-
ciation.

The difficulty of explaining former glaciations has been
greatly increased by such assumptions as that they were due to
the development of a severer climate at the same; time through-
out the world.

CLIMATIC VARIATIONS. 13

There is not vet adequate evidence that the former glaciations
were accompanied by a universal change of climate. It is true
that there is evidence of a. more extensive Pleitocene glaciation
in many regions of the world, including Mount Kenya, upon the
equator in British East Africa, Mount Koseiusko_Jn Southeas-
tern Australia, wastera Tasmania, the South Island of New
Zealand, Patagonia and a belt practically all across the tempe-
rate regions of the Northern Hemisphere. Accordingly it is
claimed, as by Ekholm (op. cit. p. 34) that the snow line was
everyvhere 1,000 metres lower at the time vhen Europe had
its i%Great Ice Age." But there are too many cases in which evi-
dence of such former extension has been sought for in vain, for
a universal lowering of temperature in the Pleistocene to be
accepted as yet finally established. In the North Island of New
Zealand, there is no evidence of any former glaciation ; and had
its existing snowfields extended more than 3,000 feet lower,
they should have left some traces of so great growth. D'Orbigny
and Whymper both failed to find1 any evidence of any greater
extension of the existing glaciers on the Equatorial Andes, than
could be explained by a local variation in the winds; in Equa-
toriaL Africa no Pleistocene glacial deposits have been found,
except on the dwindling summits of the highest mountains ; and
the coastal raised beaches give no evidence of any contemporary
reduction in the temperature of the adjacent seas. There is no
evidence of any Pleistocene glaciation on the mainland of Aus-
tralia, except on the highest summit of the Australian Alps;
and through Mount Kosciusko, which is HOAV 7256 feet above sea
level, in a region with a 60 inch rainfall, had once a few small
glaciers, there is no evidence in Australia generally of a colder
Pleitoeene climate. In fact the early Pleistocene or Pliocene
fauna of central Australia indicates the extension, then of the
tropical fauna of northern Australia into the temperate regions
of the Continent. Neither the flora or fauna of the Pleistocene
deposits of Victoria indicates a colder climate than that of the
present time.

The glaciations themselves, moreover, though often very ex-
tensive, appear to have been always local. Thus those of the
Pleistocene in the Northern Hemisphere were grouped around

14 -t. W. GREGORY.

a series of centres, which are not always in particularly high
latitudes. In North America there appear to have been three
glacial centres, that of the Canadian Rocky Mountains in lat.
55° [60°; that of eastern "Canada in lat. 50|55°, and with its
southern edge extending to 42° N. ; and that of Greenland of
which the centre is from 70|75° N.

In Europe, the glaciation of the British Isles, extended as far
south a® 52° ; that of Scandinavia, from a centre between GO0
and! 65° N., overrode the country as far south as northern
Germany in lat. 53° ; and the other centres further south deve-
loped where high mountains, such as the Alps, occurred near
warm seas.

i

y. — CAUSES OF CLIMATIC VARIATIONS.

If it be accepted that former climatic changes involve less
extreme changes of temperature than have been generally assu-
med, and that we are not called on to explain former tropical
forests in the Arctic lands, or fossil coral reefs in the Arctic
seas, or occasional universal refrigerations of the earth, then
the problem of climatic variations is greatly simplified.

1. — THE ELEVATION THEORY.

Several explanations, attractive from their simplicity, may
then be at once dismissed. The theory of the migrations of the
poles into temperate regions, although supported by Oldham
and Penck for the Upper Palaeozoic glaciation, is contradicted
by the evidence of palaeontology; and the explanations it would
give of world wide changes are not required. The once popular
theory that ice-caps have been produced by the greater elevation
of the land may be abandoned, as opposed to meteorological
principles, and as implying a reversal of the facts, glaciations
having so often accompanied periods of greater submergence1
of the land, and milder climates having coincided with periods of
emergence; and it would be quite inapplicable to the Upper
Palaeozoic glaciation of Australia, of which tire glacial deposits
were in places submarine.

CLIMATIC VARIATIONS.

2. — THE OBLIQUITY OF THE ECLIPTIC.

Nor, in spite of the fresh use made of 'it by Ekholm and
Dickson, does the variation in the obliquity~~t»f- the ecliptic
appear to help materially; for all the influences of this agency
are open to the fundamental objection that variations in obli-
quity recur at what, geologically speaking, are short and fre-
quent intervals; whereas ancient glaciations happened but sel-
dom, and were apparently irregular in their time of return.

3. — VARIATIONS IN THE CARBONIC ACID CONTEXT
OF THE ATMOSPHERE.

The view that now seems most popular explains the major cli-
matic changes by variations in the powers of selective absorp-
tion of heat by the atmosphere. The change is attributed either
to variations in the amount of aqueous vapour as urged by de
Marchi,1 or of carbon dioxide as advocated by Svante Arrhe-
nius,2 and recommended to us by the brilliant advocacy and high
authority of Prof. T. C. Chamberlin.3

The aqueous vapour theory has been adequately disposed of
by Arrlienius, whose alternative is especially attractive, as it
demands comparatively small differences of temperature and
very modest variations in the amount of carbonic acid. Thus he
calculates that an increase of the carbonic acid from .03 to .09^
would give the Polar regions a temperate climate, by a rise of
from 12° to 16° F. Nevertheless, this theory — that former colder
periods were due to a reduction of the carbonic acid in the air
and warm periods to an increase in its amount — is faced by obje-
tions which I venture to think still inadequately answered.

Xo one is likely to deny the possibility of great variations in
the former composition of our atmosphere. The theories of Ko-

1 De Marchi. "Lt- Cause dell'era Glaciale." Pavia, 1S!i.">.

- S. Arrheriius. "On the Influence of Carbonic Acid in the Air upon the Tempera-
ture of the Ground." I'hil. Mag. Ser. r>. vol. XI.I. IS'.Mi, pp. ii:!T-^7»i.

3 C. Chamberlin. "A Croup of Hypotheses bearing on Climatic Changes.'1 Journ.
Geol. Vol. V, 18JI7, pp. (J76-6S3. "The Influence of Great Epochs of Limestone Formation
upon the Constitution of the Atmosphere." Ibid. Vol. VI, 1808, pp. 600-621.

16 -T. W. GREGORY.

ene (1856), Phipson (1893-4), or Stevenson (1902)1 that the
primaveral atmosphere was many times larger than at present,
was rich in carbonic acid, and had no free oxygen, may be inap-
plicable to any part of geological time; though they may very
likely be true for the first formed atmosphere, long before the
date of the oldest known sedimentary rocks. From the earth's
surface we look up through zones of atmosphere, in which the
oxygen and carbonic acid steadily diminish, and the minute pro-
portion of hydrogen at sea level increases*, until, 50 miles high,
the air consists practically of hydrogen alone.2 The aurora fla-
res above u® in a mixture of hydrogen and neon ; and as there is
evidence of such fundamental variations in the atmosphere in
space, there may well have been marked changes in time. Then?
are so many agents pouring carbonic acid into the air, and so
many others withdrawing it, that it would be strange if the
present equilibrium had always been maintained.

(a) The Oceanic Control.

Nervertheress it must not he forgotten that the ocean, as
shewn by Schloesing,3 supported by the weighty experiments
of Dittmar, controls the amount of carbonic acid in the atmos-
phere. If the amount of carbonic acid in the atmosphere is dimi-
nished1, the bi-carbonates in the sea are dissociated; the gas thus
liberated' is1 poured into the air, until the former equilibrium
between the tension of the carbonic acid in the atmosphere and
in the sea is reestablished. Hence a reduction of carlxmie acid in
the air is automatically followed by the discharge of nearly
as large a quantity from the sea ; so that any reduction is dis-
tributed between the air and the ocean. Any increase of carbonic
acid in the atmosphere is followed by the reverse change, and
only one sixth of the amount poured into the atmosphere is
retained there. It is true that great variations in the relative

1 J. Stevenson. "The Chemical and Geological History of the Atmosphere." Phil.
Mag. ser. 5, Vol. L. pp. 312-.'W:i. :U)!)-40~. Also Pt. II, "The Composition and Kxtcnt of
the Atmosphere in very Primitive Times." Phil. Mag.. Ser. (!. Vol. IV. 1!><>U, 1>1>- 4-»x.r.l

2 Sir J. D. Dewar. "The Problems of the Atmosphere." Proc. U. lust. Vol. XVII,
1902, p. 226.

3 Schloesing. "Sur la Constance de la Proportion d'acide carbonique dans 1'alr."
Compt. Rend., Vol. 90, 1880, p. 140.

CLIMATIC VARIATIONS. 17

extent of sea and land would affect the dissociation pressure of
the bi-carbonates in the sea; but it would require a great reduc-
tion in the area of sea surface to affect the equilibrium appre-
ciably.

(b) Possible Evidence from Palaeontology.

Efforts may be made to ascertain from palaeontological evi-
dence whether the atmosphere has recently altered its composi-
tion. This line of enquiry does not promise reliable conclusions,
owing to the powers of adaptation of both animals and plants
to changes in the atmosphere. An increase in carbonic acid, pro-
vided it be not accompanied by organic pollution, from three
parts to 100 parts in 10,000— an increase ten times as great as
the maximum considered by Arrhenius — is inappreciable to
man. The ordinary data of mine ventilation and the experimen-
tal results of Dr. J. S. Haldane and Dr. Lorraine Smith, shew
that men can stand, whithout serious inconvenience, an increase
of carbonic acid to even 400 parts in the 10,000 ; and as there is
no probability of temporary variations to any such degree, a slow
increase in the carbonic acid contents of the air would1 probably
have a greater indirect effect upon animals through its action
on the temperature, than by its direct effect on respiration.

(c) Non-Coincidence of Dkites.

The main objection no the atmospheric variation theory is
that it does not explain the facts of historical geology. And geolo-
gists, as the historians of the earth, test theories whenever pos-
sible, by their agreement with contemporary records.

The influence of variations of the carbonic acid contents of
the atmosphere on temperature should affect the whole world
simultaneously.1 The change need not be the same in all lati-
tudes, as is shown by Arrhenius' tables; and also by the varia-
tion in the proportion of carbonic acid with latitude, which is
rendered probable by the evidence adduced by Letts and Blake.2

1 It is sure that according to the results of Muntz and Auhin there is at present a
difference in the amounts of Carbonic acid in the air of the northern and southern
hemispheres; they estimate the mean amount as .028% in the northern and .027% in
i IK- si uthern. This difference follows from the greater area of sea in the southern he-
misphere, which can hardly have been much greater at any previous period.

•2 K. A. Letts and It. F. Blake. "The Carbonic Anhydride of the Atmosphere." Sci.
I'roc. It. Dublin, Soc. Vol. IX, 1900, pp. 179-180.

Climatic Variations. — 3

18 -T. w. GREGORY.

Nevertheless it might be expected that corresponding positions
in the two hemispheres should be almost equally affected.

There is ho \vever no evidence of a glaciation in Europe1 in
Upper Carboniferous or Permian times corresponding to that
of South Africa or Australia — in spite of the unusually exten-
sive knowledge of the land conditions of that period. The In-
dian Glaciation of Pokaran in lat. 25° N., of Chanda in lat.
19° N. may correspond to that of South Africa from lat. -4
S. to 34° 8., or of southeastern Australia from 30° S. to 40° S.
But the general collapse of the supposed Permian glacial con-
glomerates of the English Midlands, and the unconvincing evi-
dence collected to support Carboniferous glaciation in France,
as by Julien,2 leaves us with no evidence of any refrigeration of
Europe at the date of the Gondwaualand glaciation.

Again the Upper Palaeozoic glacial deposits of south-eastern
Australia do not appear to have been 'synchronous in all the lo-
calities. The glacial deposits on the northern coast of Tasmania
have been shown by Kitsoir* to be of the age of the Mersey Coal
Measures of Tasmania, which may be correlated with Lower
or Greta Coal Measures of New South Wales. The Victorian
glacial deposits are probably on approximately the same hori-
zon, which agrees with some of those of New South Wales. But
according to David,4 there were glacial deposits in New South
Wales at the following different stages in the Permo-Carbo-
niferous.

Branxton Glacial beds in the Upper Marine Series.

Greta Coal Measures.

Shales with occasional erratics in the Lower Marine Series.

Lochinvar Glacial Beds at the Base of the Lower .Marine Sc-
ries.

Again, whatever view may be held on the controversy as to
the occurrence of warm iuterglacial periods during the Pleis-

1 There Is some evidence of glacial beds of this period on the cast of the t'ral
Mountains.

2 A. Julien. "Ant-lens glaciers de la Feriode Houillore dans le Plateau Central
de la France." Ann. Club Alpin Franeais. Vol. XXI. 1S!»4. pp. lis.

3 A. E. Kitson. "On the Occurrence of Glacial Beds at Wynyard, near Table Cape.
Tasmania." 1'roc. It. Sue. Victoria, new Ser.. Vol. XV. I'.mii, 1>. :U.

4 T. W. K. David. "Discoverey of Glaciated I'.oiilders ai I'.ase <>f Permo-CarbonJ
ferous System. I.^ehinvar. New South Wales." Journ. It. Soc. N. S. Wales. Vol. XXXI II.
1889, pp. 154-159.

CLIMATIC VARIATIONS. 19

toeene glaciation of Europe, it will be generally admitted that
considerable1 oscillations occurred in the extent of the ice. Thus
the evidence in the British Isles, strongly supports the view that
after the maximum glaciation, there was a redaction in the
extent of the ice, and then after some interVal^a-fresh advance
of valley glaciers'. And such interludes, of which in the British
Isles there may have been more than one, would appear to re-
quire considerable variations in the amount of carbonic acid in
the atmosphere, repeated Avithin a short period of time.

Weighty evidence is also given against Arrhenius' theory by
the dates of the glaciations, as they do not correspond with
those at which variations in the carbonic acid contents of the
atmosphere, would be most probable. Wide spread volcanic
eruptions offer the simplest explanation of the addition of large
volumes of carbonic acid to the atmosphere; but periods of in-
tense volcanic activity do not appear to have been always follo-
wed by glacial epochs.

The great volcanic periods — the Devonian, the Permian, the
Upper Cretaceous, the Eocene and the Oligocene, — were not
followed by marked developments of glaciers. The one coinci-
dence is in the case of the Upper Carboniferous or Permian
glaciation of (rondwanaland. The Pleistocene glaciation follo-
wed a period in which volcanic action was powerful, but was
probably less than at other periods not followed by glacial
advance.

Again with the reverse case. Periods of especially active con-
sumption of Carbonic Acid were not followed by glacial epochs.
As Professor •Chamberlain has shown the most extensive remo-
val of Carbonic Acid from the atmosphere was probably during
the formation of sheets of limestone; while coal seams contain
a smaller, but still large amount of Carbon obtained from the
carbonic acid of the air. The great limestone building periods
fixed enormous1 quantities of carbonic acid1, which must have
come from the atmosphere, because, if obtained from the sea,
its fixation must have led to the transference of a fresh supply
from the atmosphere. The greatest limestone periods are pro-
bably the Lower Carboniferous, the Jurassic, the Upper Cre-
taceous and the Eocene and the Miocene. But none of them was

20 J. W. GREGORY.

a period of active glaciation. Speaking generally, they appear
to have been warmer, than the average. Thus in the British
Isles we find unusually well developed growths of corals in the
Lower Carboniferous and the Jurassic; the British Eocene flora
included plants suggestive of a warmer climate than that of the
present time, while the marine faunas of the Middle Cainozoic
in Europe and southern Australia, indicated that those seas
were then warmer than they are to-day. The Upper Cretaceous
alone gives any indications of cold conditions, as shown by the
probably ice-borne boulders in the English chalk, and the tem-
perate aspect of ist fauna ; but the oft stated view that Greenland
then enjoyed a subtropical climate rests on evidence, which at
least does not support the idea that the period was one of
universal severity. The apparent independence of the times of li-
mestone formation and glaciation is further shown by the face
that the chief glacial periods — the Cambrian in Australia and
eastern Asia, the Upper Carboniferous or Permian of South
Africa, India and Australia, and the Pleistocene in the Northern
Hemisphere were not periods of great limestone formation.

4. — CHANGES IN TEMPERATURE GRADIENT
OF THE ATMOSPHERE.

The influence of changes in the composition of the atmosphere
is also the basis of Dickson's theory.1 But he traces its influence,
not through the variations in heat absorption by the atmosphe-
re, but through variations in the temperature gradient from the
tropics to the polar regions. Dickson's paper is of value from
its clear statement of the facts showing that a development of
glaciation is possible with only a small change in mean tem-
perature.

Pickson appeals to a former difference in the temperature
gradient between the polar and equatorial regions; he attribu-
tes the change in gradient either to the changes that are always
in progress in the obliquity of the ecliptic, or to variations in
the carbonic acid in the air. He shows that either would give

1 H. N. Dickson. "The Mean Temperature of the Atmosphere and the Causes ot
Glacial Periods." Geogr. Journ., Vol. XVIII, 1901, pp. 516-523.

CLIMATIC VARIATIONS. 21

effects of the magnitude required ; but it seems doubtful whether
either will agree whit the records of historical geology; for as
regards the first cause, the change in the obliquity is geologically
speaking a short and co'sntant oscillation ; and as to the second,
as it rests1 on the variation of carbonic acid, -it is open to the
same objetions as to those of Arrhenius' theory.

5. — CHANGES IN ATMOVSPHERIC CIRCULATION.

That the explanation of glacial periods is to be sought in
changes in the circulation of the atmosphere resulting from
geographical changes, has been several time® suggested, in
accordance with Buchan's results.1 This principle has recei-
ved ist fullest application to a specific ca^e by Harmer2 to
the Pleistocene climate of north-western Europe. And moreover
Dickson has1 shown how the distribution of the glaciations in
that case corresponds whith what would be expected, if they
were due to differences in atmospheric circulation. Such me-
teorological changes would be quite inadequate to explain the
occurrence of a tropical climate in the Arctic regions, but they
would account for changes of temperature of a few degrees, and
for glaciations by local concentrations of the snow-fall. The
difference between the climates, of western Europe and eastern
America is obviously due to meteorological conditions, resulting
from geographical position. The differences on the two coasts
of the North Atlantic were naturally first atributed1 to the in-
fluence of ocean currents ; but with our present knowledge as to
their feebleness and the ending of the Gulf Stream of Newfoun-
dland, ocean currents1 may be dismissed as a very subordinate
factor. A different distribution in air pressure resulting is a
different circulation of the wind would probably be a more
effective cause, and appear to me at present to offer the best
prospects of a satisfactory solution to the problem. It is the
only explanation that seems to agree with the essential facts,

1 For instance, I endeavoured to show in 1894, that the more extensive glaciation
of Mount Kenya was due to a local difference in the atmospheric pressure due to the
former greater height of this denuded volcano. "The Glacial Geology of Mount Kenya."
Quart. Journ. Geo. Soc. Vol. L, 1894, pp. 527-530.

2 P. W. Harmer. Quart. Journ. Geol. Soc., Vol. LVII, pp. 405-472.

22 .1. W. GREGORY.

viz, the development of glaeiation from scattered centres, and
at somewhat different dates, and the apparent independance of
the glaciation® in distant continents, and their apparent direct
dependance on a particular adjustment of meteorological con-
ditions.

The slow march of glaciation across North America and pos-
sible also across Europe, is intelligible on this hypothesis, and
there is no reason, on that theory, to expect coincidence of gla-
ciation® in the northern and southern hemispheres. The former
glacial extensions in Australasia can thus be easily explained ;
for the evidence, so far, appears to be only convincing in loca-
lities either on the edige of the Antarctic regions, or in local
areas where the meteorological conditions are unusual. New
Zealand is often quoted as having been glaciated, either in the
Pleistocene, or at the same time as the glaciation of Europe. But
it shotild be remembered that there is no evidence yet of any
glaciation in the North Island of New Zealand, and the former
range of the glaciers in the South Island has been considerably
exaggerated. On the western slope of the South Island, glaciers
in lat. 43°20' S. still come down to the level of 600 feet above the
sea; and1 it is along that coast with its intense rain-fall, that
the former ice extension is most cleary shown. In Tasmania the
Pleistocene glaciation resulted from a heavy snowfall along
the western edge of the Central Plateau, and the low moraines
yet proved, occur only in the valleys leading down to the western
coasts ; but on the mainland of Australia, the evidence of former
glaciation is very scanty. Its existence has been finally esta-
blished by the work of David and Pittman, on Kosciusko; but
the numerous cases of Pleistocene glaciation that have been as-
serted in Victoria cannot be maintained. I have visited all but
two, and saw no evidence of glacial action in any of them; and
the evidence relied on in both the places I have not seen, has
been described by others as explicable by non-glacial agen-
cies.

The Permian or Carboniferous glaciations of South Africa
India, and Australia being in low latitudes and ranging down
to sea level in New South Wales and in the Salt Range appears
at first sigth to be the most difficult problem in palaeome-

CLIMATIC VARIATIONS. 23

teorology. But the question is simplified by the following consi-
derations.

1. — The geographical conditions of the areas concerned were
very different from those of the present day.

2. — The three best known glacial centre?- occurred on tire bor-
ders of the old continent of Gondwanaland, farthest' from the
equator ; and they were probably all near mountainous country,
facing seas open to the colder zones.

3. — The only cases where the glacial deposits reached the sea,
were in the areas furthest from1 the tropics, and probably most
exposed to cold Winds.

4. — Icebergs occasionally now reach almost to the tropics;
thus in April, 1894, one was seen in the South Atlantic in lat.
26°30' S.

5. — The glacial deposits appear to have been absent from the
more tropical parts of Gondwanaland, as they disappear to-
wards the north in both Australia and in South Africa.

Both in Australia and South Africa the glaciation ocurred
in areas where mountains existed near the sea. In southeastern
Australia there is ample evidence that a wide Upper Palaezoic
sea lay to the east and a gulf to the north-west of Australia. Tn
all probability there was a large extent of land stretching
southward and cutting off the cold southern ocean from the
seas, which extended south-ward from the tropics. Under such
conditions the wind systems would have traversed the Austra-
lian lands upon a different path from that which they follow
now, and they would not have advanced so steadily. The winds
would have carried large quantities of moisture southward from
the warm northern seas, and it would have been precipitated ou
the mountains of that period, which were kept cold by southerly
winds, chilled by their passage over the former extension of
Australia to the south. In South Africa and in South America
the question is simpler, as there is no proof of the glacial depo-
sits having been laid down at sea level; they may have been
formed upon the flanks of mountain areas, kept abundantly sup-
plied with snow, by west winds blowing in from the adjacent
oceans. In India, the conditions were probably meteorologically
similar, the glaciation having been on the cooler edge of Gond-

\

24

J. W. GREGORY.

wanaland, where it was bounded by a temperate sea; and
through the glaciers ranged into the tropics in Southern India
as far .south as 17°20' N. lat. there is no proof that they occurred
•there at low levels.

It appears, therefore, probable that variations in climate,
which have l>een established on adequate evidence, can be ac-
counted for by differences in atmospheric circulation due to
different distributions of land and water. All the -evidence avai-
lable regarding the Upper Palaeozoic glaciation of Gond wa-
naland appears to be consistent with the view that the glaciers
developed, like those of the Pleistocene glaciation of North
America and of north- western Europe, in a number of scattered
localities, where mountains occurred beside the sea, and where
the meteorological produced a high snowfall and a low summer
temperature.

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